The present invention relates to a vacuum fluorescent display (VFD) panel, and more particularly to a thin film transistor (TFT) controlled VFD panel having an alkali-free glass plate therein.
The conventional TFT controlled VFD panel was reported by T. Shimojo et al. in Proceedings of the SID, vol. 29/1, pp. 65-69, 1988 under the title of "Improvement of Electrical Performance of CdSe TFTs for Application to VFDs". When TFTs as switching elements for each pixel are formed directly on an anode substrate, the anode substrate should be made of alkali-free glass. As shown in FIG. 1, a conventional TFT controlled VFD panel has TFTs 15 on an anode substrate 14 and anode-phosphor layers 16 are formed on the TFTs 15, and a grid electrode 17 and filament cathodes 18 are disposed thereabove. Leads 2 are sandwiched between the anode substrate 14 and a cover glass 5. The lead terminals 2 are connected to internal terminals (not shown). Reference numeral 22 shows a spacer member that supports and conducts current to the grid electrode 17. Moreover, the tip of the lead terminal 2 inside the cover is bent so as to be connected under compression to a wiring conductor terminal (not shown) formed on the anode substrate 14. These components have structures identical to those of the conventional VFD panel.
The TFT array on the anode substrate is mostly formed by using semiconductor materials. However, it is formed not on a soda-lime glass plate which contains and precipitates alkaline components that deteriorate the characteristics of the semiconductor materials, but on an alkali-free glass plate of quartz glass, boro-silicate glass or the like.
However, commercially available alkali-free glass plates are only those with thickness up to about 1 mm. When such a thin commercial alkali-free glass plate is used as the anode substrate and constitutes a portion of the vacuum envelope, a large size envelope cannot be achieved and thus the display area is compelled to be restricted.
In addition, if the cover glass were made of the alkali-free glass, the TFT controlled VFD panel of the conventional structure would have a problem that the fabrication cost is sharply raised and is not practical compared with the ordinary VFD panel whose enclosure consists entirely of soda-lime glass. Moreover, when a vacuum envelope is assembled by using a cover glass made of inexpensive soda-lime glass (with coefficient of linear expansion of 9.5.times.10.sup.-6 /.degree. C.) and an anode substrate made of alkali-free glass (quartz glass with coefficient of linear expansion of 0.5.times.10.sup.-6 /.degree. C.), there was a problem that the yield drops drastically due to generation of cracks and the like caused by the distortion due to the difference in the coefficients of linear expansion generated at the time of elevation or lowering of the temperature for sealing, exhaust or the like.